I have ME/CFS and always approach any claims for benefit as one who suffers from a physical disorder. If it were a mental illness treatments for such would probably improve the phsyical symptoms but it's physical. I also suffer from pre-existing depression which has become worse since I have had ME/CFS so I am on a higher dose of anit-depressants. I highly recommend the information provided by Action for ME as they have helped me in the past to claim benefits successfully and to win two appeals. Good luck!